Manufacturer and builder 5, 1869
An attempt was lately made in Philadelphia to introduce objects made of a new so-called fusible porcelain manufactured there, which, however, was nothing more than a white opaque glass, or milk-glass, as it has been sometimes called. The so-called porcelain pictures are nothing but photographs on a sheet of the glass mentioned. It is very unlike porcelain, as the latter will not crach through the action of heat, and therefore quite different from the real article. This milk-glass, on the contrary, is easily cracked by the application of a small flame, and is very easily fusible, being a lead glass, to which the opacity has been given by one or more of the following ingredients: peroxide of tin, antimonious acid, chloride of silver, or phosphate of lime. The last is the most common, and by it is prepared to so-called bone-glass. For a species of enamel known as bone-glass, jawbones of cattle and horses are preferred, because of the enamel of the teeth they contain. These bones are first thoroughly burned in a separate oven, then ground to a very fine powder, and from eight to twenty per cent of this is added to the powder prepared for common flint-glass, according to the degree of opacity or translucency require. In the melted state this glass is perfectly clear and transparent, but only becomes milky by the working. Light passed through a turbid or troubled glass of the above kind undergoes, by absorption, a change, as may be seen in some lamp-shades of this kind, and produces a kind of color-play, which is called opalescence. Such glass, when milk-while, resembles alabaster. Different tints are produced in it by minute quantities of the following substances: always bone-ashes, to produce the white translucent effect; small portions of forge-scales for green; oxide of uranium for greenish yellow; oxide of nickel, arsenious acid, etc. Oxide of tin may be used in place of the bone-ashes; but it requires considerably more to produce the same effect, and, besides, is much dearer.
In order to enamel various objects, use is made commonly of a mixture of tin and lead, which is more easily oxidized at a red heat with free access of air, than either of the two metals separately. This is caused by the affinity of the stannic acid produced for the oxide of lead. This property is made use of by manufacturers of a very opaque enamel for the purpose of enameling different objects. An oxide of tin containing lead, or an oxide of lead containing tin, is made; therefore it contains lead for the flint-glass element of the enamel, and tin oxide to render it opaque. One part of tin and from one to six of lead are brought to a dark red heat in a flat cast-iron vessel, exposed to a current of air, and stirred. After removing the remaining metallic granules by washing, it is mixed with equal parts of quarry powder, or sand, and one fourth part of potash, soda, or salt. After melting together and cooling, it is powdered and applied to the objects to be enameled with a brush. This so called burning is done in a kind of mustic-furnace, which melts the enamel over the surface, and causes it to form an equal white opaque coating on the iron or other objects heated.
In the same way as white enamel is produced from colorless glass, so colored enamels may be produced from colored glasses by the addition of the above enamel, which constitutes a beautiful department of industry not yet sufficiently explored, and for which the necessary information will be found in the next article on colored glass.